The turbulent/non-turbulent interface in wall-jets

The present study addresses the turbulent/non-turbulent interface (TNTI) in turbulent wall-jet flow using direct numerical simulation data. Three detection methods are investigated based on the vorticity, passive, and active scalars. In these methods, TNTI is detected as an isosurface of the scalar value, with a proper identification of the scalar threshold. Performance of the detection methods is assessed and compared qualitatively and quantitatively, and the most suitable one is determined. The intermittency parameter and probability density function for the location of the TNTIs are computed and analyzed using different methods. The TNTIs obtained from the passive and active scalar methods exhibit similar geometrical features, and both can reliably detect the TNTI in wall-jet flows. This is evidenced by an abrupt change in the conditional mean profile of turbulent kinetic energy and a small peak in the vorticity at the interface. While both passive and active scalar methods effectively detect the interface, the vorticity method tends to mark the interface at a higher position. The TNTI is not a simple monofractal but has a non-uniform geometry defined by a continuous spectrum of fractal dimensions. Additionally, local entrainment velocity at the TNTI is evaluated. Although detrainment is found to be more significant in turbulent wall jets compared to turbulent free jets and turbulent boundary layers, entrainment has been demonstrated to remain the dominant overall mechanism. It is demonstrated that local entrainment velocity is influenced by the interface curvature, with faster entrainment observed at regions of extreme mean curvatures, regardless of the sign.